AbstractThe performance of the first diode-pumpedYb3-doped Srs(PO4)3F (Yb:S-FAP) laser is discussed.We found the pumping dynamics and extraction cross-sections of Yb:S-FAP crystals to be similar to thosepreviously inferred by purely spectroscopic techniques.The saturation fluence for pumping was measured to be2.2 J/cm2 using three different methods based on either-the spatial, temporal, or energy transmission propertiesof a Yb:S-FAP rod. The small signal gain implies anemission cross section of 6.0x10-20 cm2 that fallswithin error bars of the previously reported value of7.3x10-20 cm2, obtained from spectroscopic techniques.Up to 1.7 J/cm3 of stored energy density was achievedin a 6x6x44 mm Yb:S-FAP amplifier rod. An InGaAsdiode array has been fabricated that has suitablespecifications for pumping a 3x3x30 mm Yb:S-FAP rod.In a free running configuration diode-pumped slopeefficiencies up to 43% were observed with outputenergies up to -0.5 J per 1 ms pulse. When the rod wasmounted -in a copper block for cooling, 13 W of averagepower was produced with power supply limitedoperation at 70 Hz and 500 ps pulses.L IntroductionYb3+ based lasers have received substantialattention over the past several years. For example, Fanand coworkers) have demonstrated efficient laser actionin Yb:YAG, while DeLoach et. al.2 have developedlasers based on Yb-doped fluoroapatite, as well asseveral of its crystalline derivatives. In addition, Hannaand coworkers have investigated Yb-doped fiberlasers. Yb-based materials, which typically lase around1 pm, are aided by the simple electronic structure ofYb3+ in that it has only two accessible electronic states,eliminating the detrimental impact of upconversion or

excited state absorption. In addition, the inherentlysmall quantum defect of -10-15% has led to relativelylarge intrinsic laser slope efficiencies. 1-3 One detrimentintrinsic to Yb3+ based lasers is that they operate asquasi 3-level systems since the terminal level can bethermally populated at room temperature. Although, ifthe pump source is sufficiently intense to effectivelybleach the ground state, the laser will operate more likea 4-level system. Laser diodes are the obvious choice asa high irradiance pump source since the -900-1000 nmpump region for Yb-doped crystals and glasses overlapswith commonly available diode structures. Theemission lifetimes of Yb-doped materials also tend to besignificantly longer (> 1 ms) than Nd3+ doped into thesame media, offering practical advantages in loweringthe cost and increasing the effectiveness of diode laserpump sources. The above considerations suggest thatdiode-pumped solid-state Yb-lasers may providesignificant advantages over Nd-lasers for certainapplications such as pulsed kJ class slab-laser systems.We believe that Yb:S-FAP is also particularlywell adapted for low to medium average power laserapplications that are sensitive to overall efficiencies.Yb:S-FAP has a relatively low pump saturationintensity of 2.0 kW/cm2 which is well suited to readilyachievable diode array irradiances. In contrast,Yb:YAG has a pump saturation intensity of 28 kW/cm2,which is more difficult to exceed, although Yb:YAGdoes offer significantly better thermal properties that aremore desirable for high average power applications.II. Yb:S-FAP crystal growthWe have developed a method for producinghigh quality S-FAP crystalline boules, up to 1" diameterby 7" long, with Yb3+ concentrations of 1.2x1019 cm-3based on Czochralski growth techniques. This